?
Solved

Tcp sequencing and IP fragmentation

Posted on 2004-09-06
6
Medium Priority
?
965 Views
Last Modified: 2013-11-29
I know TCP segments are numbered (with sequence numbers) for reassembly/error recovery reasons.  
How do TCP segments and IP fragmentation correlate?  If the sender has an IP datagram that exceeds the layer 2's MTU, it fragments the datagrams and each datagram gets sent as separate frames across the wire.

Now, what is responsible for reassembly on the other end?   I know that with packet fragmentation, a packet id is defined and the "fragment" flag is turned on.  What is TCP's job in the reassembly of fragged IP datagrams?  Arent the fragmented packets ID'd anyway?
Thanks
0
Comment
Question by:dissolved
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 3
6 Comments
 
LVL 7

Accepted Solution

by:
LimeSMJ earned 2000 total points
ID: 11991697
The Internet layer (IP protocol) within the TCP/IP architecture is responsible for reassmebly of a fragmented packet.

If a packet hits a network where the MTU is lower than the originator, the packet is fragmented to allow for transmission.  The fragmentation is done usually by the routers along the way so the sender does not know of the packet manipulation (sender won't decrease packet size if MTU is smaller).  Since the sender is only sending sequenced packet numbers (not caring about fragmentation), the fragmented packets need to have their own sequencing flags added in by the router - so that the IP layer can defragment at the receiving end.

TCP's job is provide the transmission link... IP helps negotiate the data packets along the way.




0
 

Author Comment

by:dissolved
ID: 11991843
Cant fragmentation occur between the network layer and data link layer as well?
I heard that if an IP datagram is handed to the data link layer, it will be fragmented if it is to big. And each packet leaving the network will have the fragment flag turned on.

0
 

Author Comment

by:dissolved
ID: 11991850
Also, cant the receiver send a ICMP error back to the sender saying "needs fragmentation" or something like that?
0
The Ideal Solution for Multi-Display Applications

Check out ATEN’s VS1912 12-Port DP Video Wall Media Player at InfoComm 2017. Kerri describes how easy it is to design creative video walls in asymmetric layouts and schedule detailed playlists ahead of time with its advanced scheduling feature.

 
LVL 7

Expert Comment

by:LimeSMJ
ID: 11992029
The ICMP request for fragmentation is generated by a router if the packet received has the "do not fragment" bit set and cannot be forwarded because the next hop MTU is too small for the packet (fragmenting is not possible to make the MTU smaller, so this error occurs).  The actual destination computer would never send an ICMP request for fragmentation.
0
 
LVL 7

Expert Comment

by:LimeSMJ
ID: 11992101
I am not sure what you meant by :

"...IP datagram is handed to the data link layer, it will be fragmented if it is to big..."

The IP datagram is the packet itself... the sent payload.  If the sending computer knows of potential MTU problems, it may allow the packet to be fragmented (thus preventing any transmission delays).  Unless there is a stack misconfiguration, there is no way for a datagram to be internally fragmented between the OSI layers.
0
 

Author Comment

by:dissolved
ID: 11992105
thanks
0

Featured Post

Get MongoDB database support online, now!

At Percona’s web store you can order your MongoDB database support needs in minutes. No hassles, no fuss, just pick and click. Pay online with a credit card. Handle your MongoDB database support now!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

WARNING:   If you follow the instructions here, you will wipe out your VTP and VLAN configurations.  Make sure you have backed up your switch!!! I recently had some issues with a few low-end Cisco routers (RV325) and I opened a case with Cisco TA…
This article is in regards to the Cisco QSFP-4SFP10G-CU1M cables, which are designed to uplink/downlink 40GB ports to 10GB SFP ports. I recently experienced this and found very little configuration documentation on how these are supposed to be confi…
Viewers will learn how to connect to a wireless network using the network security key. They will also learn how to access the IP address and DNS server for connections that must be done manually. After setting up a router, find the network security…
NetCrunch network monitor is a highly extensive platform for network monitoring and alert generation. In this video you'll see a live demo of NetCrunch with most notable features explained in a walk-through manner. You'll also get to know the philos…
Suggested Courses

743 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question